arXiv subjects

Chee Sheng Fong

Publications and source records attributed to Chee Sheng Fong.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic Evolution of the Standard Model Flavor Charges

Since all the Standard Model (SM) parameters have been measured to precision at the percent level or smaller, one would aim to have a framework which describes baryogenesis as precise as possible, where the measured SM parameters are used as inputs. At high temperature before the electroweak symmetry breaking, the SM contains 15 approximate $U(1)$ symmetries with the associated flavor charges. To describe the evolutions of these flavor charges in a flavor-basis-covariant manner such that physical observables are flavor-basis-independent, a total of five density matrices in flavor spaces are required: three for quarks and two for leptons. Taking into account the quantum chromodynamics and electroweak sphaleron interactions together with all the Yukawa interactions, we obtain the complete flavor-covariant Boltzmann equations. By imposing chemical equilibrium in the lepton sector, we further derive a new effective quark-flavor-covariant formalism which can be used when baryogenesis occurs through number asymmetry generation in the quark sector. To verify the consistency of this complete formalism, we first apply it to leptogenesis scenarios, reproducing previous results which utilize the effective lepton-flavor-covariant formalism up to small corrections due to the quark chemical equilibrium approximations used in the latter formalism. Then, for the first time, we apply the complete formalism as well as the effective quark-flavor-covariant formalism to a cloistered baryogenesis scenario where number asymmetries are generated in the quark sector, showcasing the good agreement between the results from the two formalisms. Finally, we release the first generic baryogenesis public code BOLEH (BaryOn and Lepton charge Evolution in the Hot big bang) where all the SM interactions are taken into account.

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arXiv2026

Seesaw Cosmology

We study perturbative reheating in which the inflaton transfers its energy to the Standard Model through right-handed neutrinos (RHNs) responsible for light-neutrino masses via the type-I seesaw mechanism. We refer to the resulting nonstandard thermal history as $seesaw$ $cosmology$. When produced relativistically and sufficiently long lived, the RHNs generate a characteristic sequence of inflaton, relativistic-RHN, nonrelativistic-RHN, and Standard Model radiation domination. We solve the Boltzmann system while retaining the production-time dependence of the nonthermal RHN distribution and its relativistic-to-nonrelativistic transition. The Standard Model temperature rapidly approaches a plateau during inflaton domination and subsequently scales as $a^{-1/4}$ and $a^{-3/8}$ during relativistic- and nonrelativistic-RHN domination, respectively. We investigate the implications of seesaw cosmology for dark-matter production. Direct production through inflaton decays can be enhanced relative to conventional reheating by a factor of order $m_\phi/(2m_N)$, while ultraviolet freeze-in exhibits the critical temperature powers $p = 12$ and $20$, leading to potentially large contributions before the final radiation-dominated era. Seesaw cosmology therefore connects neutrino-mass generation, the pre-BBN thermal history and phenomena such as dark-matter production and baryogenesis.

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arXiv2026

Towards Precision Neutrino Fits in GUTs: Relevance of One-Loop Finite Corrections

In this work, we perform a dedicated analysis of fermion mass fits in the minimal $SO(10)$ grand unified theory (GUT), going beyond the tree-level approximation by incorporating one-loop finite corrections to the neutrino mass matrix. We show that parameter regions that successfully reproduce all fermion masses and mixings at tree level can lead to significant deviations in neutrino masses and leptonic mixing parameters once the radiative corrections are included. These results expose a limitation of conventional tree-level fitting procedures and highlight the sensitivity of neutrino observables to loop effects. Since in the minimal $SO(10)$ GUT the same set of Yukawa parameters simultaneously governs quark masses, charged lepton masses, and neutrino properties, these radiative corrections propagate across all fermion sectors, reshaping the viable parameter space in a highly non-trivial and correlated manner. We find that the largest corrections to the masses and mixing angles are of order $\mathcal{O}(30\%)$-$\mathcal{O}(40\%)$, therefore, cannot be neglected. In light of the current precision of neutrino oscillation measurements, and the expected improvements from ongoing and future experiments, we demonstrate that the inclusion of one-loop effects is essential for a consistent and reliable exploration of the parameter space, with important implications for the predictivity of $SO(10)$ GUTs.

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arXiv2025

$N_{\textrm{eff}}$ Constraint on Pseudo-Dirac Neutrinos

After the electroweak symmetry breaking, we can write down two types of mass for the Standard Model neutrinos, Dirac or Majorana. It is often said that both types of mass cannot be distinguished in neutrino oscillation phenomena. This is in fact not true if neutrinos are pseudo-Dirac (strictly speaking still Majorana) where they mix almost maximally with sterile neutrinos to form pseudo-Dirac pairs. If this is indeed realized in Nature, what we observe experimentally as three mass eigenstates are actually three pairs of mass eigenstates with yet-to-be-measured new mass splitting among each pair. While the new mass squared splitting of the first and second mass eigenstates have stringent constraints from solar neutrino to be $|\delta m_{1,2}^2| \lesssim10^{-11}\,\textrm{eV}^{2}$, the one regarding the third mass eigenstate has a weaker constraint $|\delta m_3^2| \lesssim10^{-5}\,\textrm{eV}^{2}$. By keeping only one nonzero pseudo-Dirac mass squared splitting at a time, we derive an effective 3+1 description for the pseudo-Dirac scenario. Then we use the Cosmic Microwave Background (CMB) constraint on neutrino relativistic degrees of freedom $N_{\textrm{eff}}$ to derive a new constraint $|\delta m_3^2| < 2 \times 10^{-6}\,{\rm eV}^2$ and show that the future CMB-S4 and CMB-HD can improve this bound by an order of magnitude.

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arXiv2025

One Phase to Rule Them All: Spontaneous CP Violation and Leptogenesis in SO(10)

We present a renormalizable $SO(10)$ grand unified theory with a minimal Yukawa sector consisting of a $126_H$, a real $10_H$ and a real $120_H$, where $CP$ violation has a spontaneous origin. We show that the Yukawa sector in this setup, which consists of only 19 real parameters, is capable of simultaneously reproducing the observed fermion masses and mixings, including neutrino oscillations, as well as the baryon asymmetry of the Universe via thermal leptogenesis. In this framework, $CP$ is spontaneously broken when a $CP$-odd Higgs field $54_H$, used for GUT symmetry breaking, acquires a non-zero vacuum expectation value. All $CP$-violating phases, including the Dirac phases $\delta_\mathrm{CKM}$ and $\delta_\mathrm{PMNS}$, the neutrino Majorana phases, as well as those responsible for leptogenesis, arise solely from a single complex parameter in the Higgs potential. The proposed minimal setup predicts a normal ordering of neutrino masses, with the atmospheric mixing angle $\theta_{23}$ preferred in the first octant and the leptonic $\delta_{\mathrm{PMNS}}$ lying in the range $(-38^\circ,\, +31^\circ)$. The fermion fits in our scenario further yield a strongly hierarchical mass spectrum for the three right-handed neutrinos, $(M_1,\,M_2,\,M_3) \sim \left(10^{5},\, 10^{12},\, 5\cdot 10^{14}\right)~\mathrm{GeV}$, which is shown to result in successful $N_2$-dominated leptogenesis, consistent with current cosmological data.

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arXiv2025

Electroweak Triplet Scalar Contribution to $SO(10)$ Leptogenesis

We show that electroweak triplet scalar can significantly impact baryogenesis via leptogenesis in concrete and predictive $SO(10)$ GUTs, even when light neutrino masses arise predominantly from the type-I seesaw mechanism. This is illustrated within a minimal renormalisable $SO(10)$ model with $\mathbf{10}$ and $\overline{\mathbf{126}}$ scalars in the Yukawa sector and a global Peccei-Quinn-like symmetry. The quark-lepton unification and the flavour structure of the fundamental Yukawa couplings enforce type-I dominance in the light neutrino masses, also suppressing the triplet-induced CP asymmetries in the right-handed neutrino decays. However, the triplet's own decays introduce a new CP-violating source, which can enhance or suppress the total baryon asymmetry. For triplet mass near the right-handed neutrino mass scale, this contribution can dominate, making it essential in assessing the viability of $SO(10)$ leptogenesis scenarios.

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arXiv2024

Probing low-reheating scenarios with minimal freeze-in dark matter

The parameter space of freeze-in dark matter (DM) with mass $m_\chi$ through light dark photon (``minimal freeze-in DM'') is currently being probed by direct detection experiments through electron and nuclear recoil. Exploring the DM production in the mass range $10^{-2}~{\rm MeV} < m_\chi < 10^3$ TeV, we quantify the impact of quantum statistics and the reheating dynamics (beyond the instantaneous reheating approximation) on the DM production in the early universe, in particular, the dependence on the cosmic equation of state and the scaling of the temperature of the Standard Model bath during reheating. Special cases corresponding to matter-domination and kination are carefully studied. To fit the entire observed DM relic abundance, low-temperature reheating scenarios require an increase in the coupling between dark and visible sectors which, in turn, enhances the regions of the parameter space that are already tested and will be probed by next-generation direct detection experiments for diverse reheating scenarios.

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arXiv2024

Conformal Leptogenesis in Composite Higgs Models

We study the generation of the baryon asymmetry in Composite Higgs models with partial compositeness of the Standard Model (SM) fermions and heavy right-handed neutrinos, developing for the first time a complete picture of leptogenesis in that setup. The asymmetry is induced by the out of equilibrium decays of the heavy right-handed neutrinos into a plasma of the nearly conformal field theory (CFT), i.e. the deconfined phase of the Composite Higgs dynamics. This exotic mechanism, which we call Conformal Leptogenesis, admits a reliable description in terms of a set of ``Boltzmann equations'' whose coefficients can be expressed in terms of correlation functions of the CFT. The asymmetry thus generated is subsequently affected by the supercooling resulting from the confining phase transition of the strong Higgs sector as well as by the washout induced by the resonances formed after the transition. Nevertheless, a qualitative description of the latter effects suggests that conformal leptogenesis can successfully reproduce the observed baryon asymmetry in a wide region of parameter space. A distinctive signature of our scenarios is a sizable compositeness for all the generations of SM neutrinos, which is currently consistent with all constraints but may be within reach of future colliders.

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arXiv2024

Leptogenesis in SO(10) with Minimal Yukawa sector

In prior studies, a very minimal Yukawa sector within the $SO(10)$ Grand Unified Theory framework has been identified, comprising of Higgs fields belonging to a real $10_H$, a real $120_H$, and a $\overline{126}_H$ dimensional representations. In this work, within this minimal framework, we have obtained fits to fermion masses and mixings while successfully reproducing the cosmological baryon asymmetry via leptogenesis.The right-handed neutrino ($N_i$) mass spectrum obtained from the fit is strongly hierarchical, suggesting that $B-L$ asymmetry is dominantly produced from $N_2$ dynamics while $N_1$ is responsible for erasing the excess asymmetry. With this rather constrained Yukawa sector, fits are obtained both for normal and inverted ordered neutrino mass spectra, consistent with leptonic CP-violating phase $\delta_\mathrm{CP}$ indicated by global fits of neutrino oscillation data, while also satisfying the current limits from neutrinoless double beta decay experiments. In particular, the the leptonic CP-violating phase has a preference to be in the range $\delta_\mathrm{CP}\simeq (230-300)^\circ$. We also show the consistency of the framework with gauge coupling unification and proton lifetime limits.

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arXiv2024

Constraining the pseudo-Dirac nature of neutrinos using astrophysical neutrino flavor data

The three Standard Model neutrinos can have Majorana mass or strictly Dirac mass, but both scenarios are practically indistinguishable in neutrino oscillation experiments. If they are pseudo-Dirac, however, there will be new mass splittings among the pseudo-Dirac pairs, potentially leaving traces in neutrino oscillation phenomena. In this work, we use flavor ratios of astrophysical neutrinos to discriminate different possible mass spectra of pseudo-Dirac neutrinos. We show that it will be possible to impose robust bounds of order $\delta m^2_3 \lesssim 10^{-12}$ $\text{eV}^2$ on the new mass squared splitting involving the third pseudo-Dirac mass eigenstates (those with the least electron flavor composition) with the future experiment IceCube-Gen2. The derived sensitivity is robust because it only assumes an extragalactic origin for the astrophysical neutrinos and hierarchical pseudo-Dirac mass spectrum. In case the neutrino sources are known in the future, such bounds can potentially improve by up to five orders of magnitude, reaching $\delta m^2_3 \lesssim 10^{-17}$ $\text{eV}^2$.

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arXiv2024

Sphaleron portal baryogenesis

Nontrivial topological vacua of non-Abelian gauge symmetry $SU(3)\times SU(2)_{L}$ of the Standard Model play an important role in baryogenesis. In particular, the baryon (and lepton) number violation from $SU(2)_{L}$ sphaleron is a crucial ingredient for baryogenesis at weak scale or higher. In this work, we point out that generically, a baryon asymmetry is induced by an asymmetry generated in the new sector through strong $SU(3)$ and/or weak $SU(2)_{L}$ sphaleron portals and vice versa. In the standard radiation-dominated early Universe, due to phenomenological constraints, the sphaleron portal baryogenesis has to take place at cosmic temperature $T\gtrsim10^{6}-10^{8}$ GeV together with a $(B-L)$-violating source. As an example, we show an explicit model where strong sphaleron portal baryogenesis occurs at the scale of Peccei-Quinn breaking to solve the strong CP problem and this coincides nicely with the scale where the Weinberg operator responsible for Majorana neutrino mass is in equilibrium.

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arXiv2023

Primordial non-Gaussianity as a probe of seesaw and leptogenesis

We present the possibility that the seesaw mechanism and nonthermal leptogenesis can be {investigated} via primordial non-Gaussianities in the context of a majoron curvaton model. Originating as a massless Nambu-Goldstone boson from the spontaneous breaking of the global baryon ($B$) minus lepton ($L$) number symmetry at a scale $v_{B-L}$, majoron becomes massive when it couples to a new confining sector through anomaly. Acting as a curvaton, majoron produces the observed red-tilted curvature power spectrum without relying on any inflaton contribution, and its decay in the post-inflationary era gives rise to a nonthermal population of right-handed neutrinos that participate in leptogenesis. A distinctive feature of the mechanism is the generation of observable non-Gaussianity, {in the parameter space where the red-tilted power spectrum and sufficient baryon asymmetry are produced.} We {find} that the non-Gaussianity parameter $f_{\rm NL} \gtrsim \mathcal{O} (0.1)$ is produced for high-scale seesaw ($v_{B-L}$ at $\mathcal{O}(10^{14-17})$ GeV) and leptogenesis ($M_1 \gtrsim \mathcal{O}(10^6)$ GeV) where the latter represents the lightest right-handed neutrino mass. While the current bounds on local non-Gaussianity excludes some part of parameter space, the rest can be fully probed by future experiments like CMB-S4, LSST, and 21 cm tomography.

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arXiv2023

New limits on $W_R$ from meson decays

In this letter we show that pseudoscalar meson leptonic decay data can be used to set stringent limits on the mass $m_{W_R}$ of a right-handed vector boson, such as the one that appears in left-right symmetric models. We have shown that for a heavy neutrino with a mass $m_N$ in the range $50<m_N/{\rm MeV} <1900$ one can constraint $m_{W_R} \lesssim (4-19)$ TeV at 90 % CL. This provides the most stringent experimental limits on the $W_R$ mass to date.

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arXiv2023

Theoretical Aspect of Nonunitarity in Neutrino Oscillation

Nonunitarity can arise in neutrino oscillation when the matrix with elements $\mathbf{U}_{αi}$ which relate the neutrino flavor $α$ and mass $i$ eigenstates is not unitary when sum over the kinematically accessible mass eigenstates or over the three Standard Model flavors. We review how high scale nonunitarity arises after integrating out new physics which is not accessible in neutrino oscillation experiments. In particular, we stress that high scale unitarity violation is only apparent and what happens is that the neutrino flavor states become nonorthogonal due to new physics. Since the flavor space is complete, unitarity has to be preserved in time evolution and that the probabilities of a flavor state oscillates to all possible flavor states always sum up to unity. We highlight the need to modify the expression of probability to preserve unitarity when the flavor states are nonorthogonal. We will continue to call this high scale unitarity violation in reference to a nonunitary $\mathbf{U}$. We contrast this to the low scale nonunitarity scenario in which there are new states accessible in neutrino oscillation experiments but the oscillations involving these states are fast enough such that they are averaged out. We further derive analytical formula for the neutrino oscillation amplitude involving $N$ neutrino flavors without assuming a unitarity $\mathbf{U}$ which allows us to prove a theorem that if $\left(\mathbf{U}\mathbf{U}^{\dagger}\right)_{αβ}=0$ for all $α\neqβ$, then the neutrino oscillation probability in an arbitrary matter potential is indistinguishable from the unitarity scenario. Independently of matter potential, while nonunitarity effects for high scale nonunitarity scenario disappear as $\left(\mathbf{U}\mathbf{U}^{\dagger}\right)_{αβ}\to 0$ for all $α\neqβ$, low scale nonunitarity effects can remain.

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arXiv2023

New Physics in Neutrino Oscillation: Nonunitarity or Nonorthogonality?

Neutrino oscillation phenomenon is a definite evidence of physics beyond the Standard Model (SM) and high precision measurement of neutrino properties will certainly give us clue about what lies beyond the SM. In particular, precise measurements of the mixing matrix elements $U_{αi}$ which relate the neutrino flavor $α$ and mass $i$ eigenstates are crucial since new physics at scale beyond experimental reach can lead to a nonunitary $U$. This in turns results in nonorthogonal neutrino flavor states. How to calculate the oscillation probability in this scenario is an important theoretical issue that will be treated here. We show that probability constructed using theory of projection probability will ensure that the theory remains unitary in time evolution and the probabilities of neutrino of certain flavor being detected as all possible flavor states always sum up to unity. This result is crucial for discovery of new physics through neutrino oscillation phenomena.

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arXiv2023

Analytic Neutrino Oscillation Probabilities

In the work, we derive exact analytic expressions for $(3+N)$-flavor neutrino oscillation probabilities in an arbitrary matter potential in term of matrix elements and eigenvalues of the Hamiltonian. With the analytic expressions, we demonstrate that nonunitary and nonstandard neutrino interaction scenarios are physically distinct: they satisfy different identities and can in principle be distinguished experimentally. The analytic expressions are implemented in a public code NuProbe, a tool for probing new physics through neutrino oscillations.

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arXiv2022

BBN Photodisintegration Constraints on Gravitationally Produced Vector Bosons

Gravitational production of massive particles due to cosmic expansion can be significant during the inflationary and reheating period of the Universe. If the particle also has non-gravitational interactions that do not significantly affect its production, numerous observational probes open up, including cosmological probes. In this work, we focus on the gravitational production of light vector bosons that couple feebly to the Standard Model (SM) particles. Due to the very feeble coupling, the light vector bosons never reach thermal equilibrium, and if the Hubble scale at the end of inflation is above $10^8$ GeV, the gravitational production can overwhelm the thermal production via the freeze-in mechanism by many orders of magnitude. As a result, much stronger constraints from the Big Bang Nucleosynthesis (BBN) can be placed on the lifetime and mass of the vector bosons compared to the scenario where only thermal production is considered. As an example, we study the sub-GeV scale dark photons, which couple to the SM only through kinetic mixing, and derive constraints on the mass and kinetic mixing parameter of the dark photon from the photodisintegration effects on the light element abundances relevant at the end of the BBN when the cosmic age was around $10^4$ s.

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arXiv2022

Snowmass Theory Frontier: Astrophysics and Cosmology

We summarize progress made in theoretical astrophysics and cosmology over the past decade and areas of interest for the coming decade. This Report is prepared as the TF09 "Astrophysics and Cosmology" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

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